Yihenew G. Selassie, Amnat Suwanarit, Chairerk Suwannarat and Ed Sarobol 2003. Equations for Estimating Nitrogen Fertilizer Requirements from Soil Analysis for Maize (Zea Mays L.) Grown on Alfisols of Northwestern Ethiopia. Agriculture and Natural Resources. 37, 2 (Jun. 2003), 157–167.